1. Field of the Invention
This invention relates generally to a tuner for use with a television receiver, and more particularly to a tuner for use with a television tuner which is provided with an improved mixer so as to avoid a beat component.
2. Description of the Prior Art
In the art, when a specified television channel is received by a television receiver, the mixer circuit of the tuner in the television receiver may produce a beat component in the intermediate frequency band due to the relationship between certain carrier frequencies and local oscillation frequencies. Hence a beat interference appears on the picture screen of the television receiver. This is a special problem in the USA because of certain frequency allocations and television standards.
United States standards place the video intermediate frequency at 45.75 MH.sub.z and the voice intermediate frequency at 41.25 MH.sub.z. When turning channel 6 which as a video carrier frequency f.sub.P6 at 83.25 MH.sub.z and a voice carrier frequency f.sub.S6 at 87.75 MH.sub.z, the local oscillation frequency f.sub.L6 is 129 MH.sub.z. A so-called 6 channel beat is produced which fall within the intermediate frequency band. The channel 6 beat is produced as follows: ##EQU1##
Also, when receiving channel in an area where channel 5 is also broadcast, since the video carrier frequency f.sub.P5 of channel 5 is 77.25 MH.sub.z and the channel 4 local oscillation frequency f.sub.L4 is 113 MH.sub.z, a so-called channel 4 beat is produced which falls within the intermediate frequency band. The channel 4 beat is produced as follows: ##EQU2##
An example of a prior art wide band mixer circuit is shown in FIG. 1. In the prior art mixer circuit shown in FIG. 1, a radio frequency signal and a local oscillation signal are applied to an input terminal 1, and are then mixed in a transistor 2 to produce sum and difference frequencies including an intermediate frequency signal. The intermediate frequency signal is amplified during mixing in transistor 2 and then applied through an emitter follower transistor 3 to a low pass peaking circuit 5 formed of a capacitor 4, a coil 8 and a capacitor 9. The peaking circuit 5 serves to pass the signals in intermediate frequency range of from 41 to 47 MHz and to reject the passage of spurious sum and difference signals outside the intermediate frequency range which exist in the output from emitter follower transistor 3. The signal from the peaking circuit 5 is fed to a transistor 7 for further amplifications and is then delivered to an output terminal 10. In this prior art example, the transistors 2, 3, 7 and the capacitor 9 are formed as part of an integrated circuit to which only the capacitor 4 and the coil 8 are externally connected.
The prior art wide band mixer circuit shown in FIG. 1 has the problem that production of channel 4 and channel 6 beat interference can not be positively prevented. That is, in the mixer circuit of FIG. 1, in the presence of strong input signals, emitter follower transistor 3 may be driven into non linear operation. The sum and difference frequencies in the output of mixer transistor 2 may be further mixed by this non linear operation to produce the above-mentioned beat interference. Since the beat interference generated by non-linear operation of transistor 3 is within the intermediate frequency range, the peaking circuit 5 is ineffective to block it.
The attenuation characteristic of the peaking circuit 5 is not very sharp as shown in the graph of FIG. 4 by a one-dot chain line curve 14a. Therefore, spurious signal components from either transistor 2 or 3 which have frequencies higher than the intermediate frequency band such as f.sub.P6 +f.sub.S6, f.sub.L6, 2f.sub.P5 and f.sub.L4, are not strongly attenuated.
These high frequency spurious signals are applied at rather high levels to the base of transistor 7. These spurious signals can drive transistor 7 into non-linear operation and hence the above-mentioned beat is produced. The beat interference ratio characteristic of the prior art mixer circuit of FIG. 1 is shown in the graph of FIG. 6 by a one-dot chain line curve 16a. The beat interference ratio is defined as the intermediate frequency signal voltage/beat signal voltage.